Prof. Challis on Newton's " Foundation of all Philosophy " 281 



meaning, and how far it can be accepted as true. I will state 

 at once that I have been induced to enter upon this inquiry 

 because it will give occasion for discussing the- characteristics 

 ^nd mutual relations of the theoretical and experimental depart- 

 ments of natural philosophy, — a discussion which the existing 

 state of physical science urgently demands, much confusion of 

 thought prevailing as to the principles, objects, and limits of the 

 two departments, and in particular as to what is fundamental 

 in theoretical research. If in consequence of the inquiry I should 

 be led to criticise somewhat freely certain views of my contem- 

 poraries, T trust that I shall be considered to be influenced by 

 no other personal feeling than the very natural desire to gain 

 acceptance for the mathematical principles of natural philosophy 

 which I have now for a long time urged on the attention of 

 mathematicians, and which I consider to be indispensable to the 

 progress of theoretical physics. I have reason to say that the 

 physical theories I have proposed have not received the atten- 

 tion which is requisite for forming a judgment on their merits, 

 chiefly because the minds of physicists are prepossessed with a 

 totally different order of ideas, of comparatively recent growth, 

 resting on arbitrary hypotheses, and having no reference to what, 

 according to Newton, is the foundation of philosophy. This 

 being the case, no course is left me but that of exposing the 

 insecurity of the opposite views. < ,Xv. 



The dictum of Newton above quoted is preceded by an enume- 

 ration of those qualities of bodies which are to be regarded as 

 " universal," and which admit of neither increase nor diminu- 

 tion (quae intendi et remitti nequeunt). After premising the 

 caution that, in regard to such quantities, fancies are not to be 

 formed (somnia non confingenda), nor the analogy of nature as 

 gathered from experience to be departed from, he proceeds as 

 follows : — " The extension of bodies is only known by the senses, 

 nor is it sensibly perceived in all ; but because it belongs to all 

 sensible bodies, it is affirmed of bodies universally. That various 

 bodies are hard, we know by experience. But the hardness of 

 the whole arises from the hardness of the parts ; and hence not 

 only of those bodies which are perceived by the senses, but also 

 of all others, we justly conclude that the undivided particles are 

 hard. That all bodies are impenetrable we gather, not from 

 reasoning, but from sensation. Those which we handle are 

 found to be impenetrable, and thence we conclude that impenetra- 

 bility is a property of all bodies. That all bodies are moveable, 

 and by a certain force, which we call vis inertia, continue in a 

 state of motion or rest, we gather from these properties of visible 

 bodies. The extension, hardness, impenetrability, mobility, and 

 vis inertia of the whole arises from the extension, hardness, im- 



